It is known to provide implanted drug infusion pumps to deliver a controlled, sustained dosage of beneficial agent or drug to the living human body. Such infusion pumps are generally classified as fixed rate infusion pumps or variable rate infusion pumps. Fixed rate infusion pumps deliver drug-carrying fluid at a preset flow rate, which cannot be changed after manufacture. Variable flow rate implantable pumps permit adjustment of the flow rate, but only prior to implantation. Readjustment of the flow rate of variable rate pumps requires removal of the pump from the patient's body and related surgery. Because of the surgical intrusiveness typically required for flow rate changes for both fixed rate and adjustable rate pumps, there has developed a trend towards selectable-rate pumps, which permit flow adjustment while the pump remains implanted in the living body.
Flow control in selectable rate pumps, however, is complicated by the need to limit or minimize the power consumption. To this end, there have been efforts to provide passive flow control elements on selectable rate pumps for minimizing power consumption while providing flow control.
For example, U.S. Pat. No. 5,820,589, to Torgerson and McMullen, the subject matter of which is incorporated herein, in its entirety, discloses the concept of an implantable pump, which is provided with a passive regulator in the form of a manifold communicating with a restrictor network, with a number (n) of bi-stable valves with two flow states, or a number (n) of multi-stable valves with a number (m) of flow states. With the bi-stable valve configuration, the combination overall allows for 2n flow rate options. With the multi-stable valves, the system has mn flow rate options. Ideally, such bi-stable or multi-stable valves, which can be referred to as passive flow rate control elements, would have no requirement for power except during flow state changes. Power is typically provided via RF signal with suitable electronic implements provided on the pump for providing an induced voltage from the RF signal. While such known passive flow control systems provide variability in flow rates, the number of attainable flow configurations is somewhat limited. It would therefore be desirable to provide an implantable pump with a passive flow control system which provides increased adjustability in flow rates over known systems.
In drug infusion applications, it is frequently desirable to provide for the introduction of a drug bolus to the patient. Bolus dosage may be required, for example, when a patient's activity results in increased pain that is not adequately controlled with normal dosage. Known passive control systems do not provide for the administering of a drug bolus. Thus, there is a need to provide an implantable pump with a passive flow control system which permits the metering and delivery of a bolus of drug.